The purpose of this project is to identify specific dimensions of cognition that are most sensitive to the functional decline that occurs in Huntington's disease (HD) using neuropsychological tests with known neural substrates. A long- term objective is to utilize the findings to develop a comprehensive is to utilize the findings to develop a comprehensive rating scale of dementia severity that is sensitive to the types of cognitive changes that occur in basal ganglia disorders, such as HD. Specifically, this project seeks to study the progression of executive dysfunction and visuospatial memory deficits in HD gene-carriers. To this end, pre-symptomatic HD gene- carriers (n=20) as well as symptomatic individuals in early and in advanced stages of HD (n=24) will be recruited. Data from these participants will be compared to data collected from age, education, and sex matched, healthy controls (n=44). The specific tests are sensitive to the cognitive changes that occur in the presence of a basal ganglia disorder, and the neural mechanisms underlying these tests are fairly well established. The intent of the study is to correlate the cognitive deterioration inherent in HD with currently used measures of functional capacity, independence, dementia severity, and behavioral abnormalities to determine which cognitive abilities best predict functional decline. Furthermore, analyses will determine whether individuals with HD demonstrate qualitatively different types of errors on the Cambridge Neuropsychological Test Automated Battery (CANTAB) tasks that may provide information on the function of the basal ganglia separate from the frontal cortex. A secondary goal of the study is to determine whether pre- symptomatic, gene-carriers exhibit cognitive deficits as they near their estimated age of onset. The presence or absence of cognitive deficits in HD prior to its neurological diagnosis is a hotly debated topic, and it is hoped that by examining how near to the estimated age of onset these deficits are observed, this study can clarify some of the contradictory findings reported in the literature. Findings may also provide data that can be used to determine when an individual is beginning to exhibit cognitive signs of HD in anticipation of a treatment that may be successful in slowing or stopping the symptoms of this disease.